Retrieving "Helicity Suppression" from the archives

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1. Pion

   Linked via "helicity suppression"

   $$\pi^\pm \to e^\pm + \nu_e \quad (\approx 0.113\%)$$
   The branching ratio for electron decay is suppressed relative to muon decay due to helicity suppression associated with the interaction of the axial vector current, which demands that the emitted neutrino must possess an angular momentum configuration that is highly unfavorable in the low-energy limit [1].
   Neutral Pion ($\pi^0$)

2. Weak Decay

   Linked via "helicity suppression"

   The Weak Nuclear Force is the only known force that violates maximal parity ($\mathcal{P}$) symmetry. This violation is intrinsic to the charged current coupling, which preferentially couples to left-handed fermions and right-handed anti-fermions.
   For leptonic final states, this leads to the universal observation that neutrinos are strictly left-handed, and [antineutrinos](/entries/anti…